The two principle pluses with 100% carbon masts is that they are notably lighter and they offer far quicker/livelier reflex response. These factors maximize a sail's performance responding to the gusts, thereby offering optimum power and drive while letting the sail breathe to the best of its ability.

The distinct advantage that an RDM has over an SDM is a greater durability in a surf environment due to its greater wall thickness. However, that's not to say that an RDM offers better performance characteristics. The two distinct mast types do offer subtle differences even if they have the exact same bend curve, and that's related to the differing overall shape of the tube structures.

What's the best? That's hard to say because folks have different personal preferences. Also, much depends on the type of sails being used, to include the intent and focus of the sail's designer. Needless to say, it's best to use the mast type/brand specified by the designer.

Overall, if you can afford it, always go for the highest carbon content, regardless whether it's an RDM or SDM.

The very extensive Boards UK magazine of last year found that Skinny s improved the performance of AMOST all sails.
Totaly agree with steveC take, and myself found some sails are better with a regular dia mast, especially 460 and above and for sure 490. The Reduced mast will hurt more when you get hit in the head (wear a helmet and laugh) sink faster, rig faster thru the mast sleeve. You will have to purchase skinny mast extensions.
HotSails Maui has the HotRod skinny, and Powerex are both very highly rated. If the size was 430 and below I would get the skinny, 460 above get as much carbon as you can afford. Some sail makers design for skinny (Ezzy) some say both will work.
Cheers

If you're past "getting in the footstraps" stage, you'll be amazed of the performance difference between 50% and a 100% mast. Rig will be lighter, more responsive, more stable, everything. Go try one.
Yes it's more sensitive to knocks/bumps, but not drastically so. The only con is the price usually.

I would certainly try any sails you intend to use on the 100% masts.(Before you buy)
Few seasons ago I upgraded (sort of) to 100% (SDM) (From C45)masts for sails 6 to 8.5.(Mixture of Tushingham and Pryde)
Found sails felt very hard and showed no improvement in speeds (GPS) . My 7 metre T Bird actually felt nicer on old C45. The 100% mast also came with a warning not to use in waves (How do you avoid them at coast ???)

Ended up selling the 2 100% masts and settling on C75`s (Tush)
Sails feel loads nicer and I have more confidence in rough water.Yep they might be a touch heavier but ???
I think on really big sails (9 plus) you might be better off with 100 and its stiffness/lightness but I reckon sails under 8metre are still designed around cheaper (sort of) masts.

Think its a different matter for RDM.430 and below I`ve gone RDM (100%) 460 and 490 have 75%`s.RDM needs 100% to get its stiffnes. All sails seem fine on this set up.

There is a lot of Hype around 100%(SDM) masts. I was very disappointed,but my biggest sail is 8.5.

I have to admit to still using my 400cm 30% carbon Windwing (Dynafiber) for my older 4.2 and 5.0 sails. Even though I bought the mast back in 1995, it has been an awesome product. I used Fiberspars for many years after retiring the old 400 around 2000 or so, but I been obliged to bring it back to life in 2005 due to mast breakages. Because it was the recommended bend curve for my sails both then and now, its viability has been without flaw, although it's a bit stiffer due the luff shape of later sails. Nevertheless, it performs incredibly well.

Also, I've had great luck with a 430cm 75% carbon Fiberspar. I've had the mast since 1999, and it's been a great product that's earned its spot in the van.

However, I'm using new 100% carbon RDMs for all newer sails all the way up to 490cm. The newer masts are totally right and more durable, and I'm good with my outlook for the future. The current performance is simply right on.

Really, I think that I'm better prepared for the future than ever before. I'm definitely stoked!

As is indicated in the posts above, the number one thing with masts is that the sail you have fit them. Unfortunately, this can not be evaluated by just looking at IMCS numbers and such things. You have to try it out or maybe listen to somebody that has tried the particular combination out.

As for carbon masts being more sensitive, yes they can be, but it is in fact not so much the carbon itself being so much more sensitive. Th "problem" is that the stiffness of carbon is so high that you can use so little of it and still make the mast the right stiffness. Then, each single little fiber carries a bigger percentage of the load and hence any single litte damage will harm the mast more.

If you follow that logic, it is not hard to see that 100%rdm masts are actually not (necessarily) very sensitive since here you use a smaller diameter to lessen stiffness and hence be able to add more material. There are 90-100% carbon rdm that are stronger than ANY sdm and also less sensitive to mistreatment off water.

But just as with sdms, when if you try to make an rdm super light (ie go significantly lower than say 1.5kg for a 4m mast) you will again approach the limit were the mast will become sensitive and not very strong.

Its not only the weight though. Designing and constructing a mast is very difficult. Also seemingly very similar mast can be very different when it comes to strength. Its all about eliminating weak spots.

I partly agree with Floyd (btw I also use Tush 400 75% mast and like it very much). Depending on your intended use/sail type, there maybe be significant improvement though. Especially with bigger race/freerace cammed sails, I think it's usually the case.
Matching bend curve from another manufacturer (although IMCS is nominally the same), can be a nightmare. Have you seen this, please help if you've used an Arrows: